In order to reduce the formation of air polluting emissions, such as NOx, combustors in a gas turbine often include a lean-premixed combustion system, wherein fuel and air are mixed in a plurality of premixed fuel nozzle assemblies disposed upstream of a combustion chamber in the combustor. However, the use of a lean-premixed combustion system also increases the propensity for flashback events, which occur when the flame within the combustion chamber flashes upstream into the premixing zone of the fuel nozzle assembly. The likelihood of flashback events occurring may be increased further when highly reactive fuels are used to fuel a gas turbine, such as hydrogen augmented fuels and fuels derived from liquefied natural gas. These flashback events often lead to flame holding, wherein the flame “holds” or remains supported within the fuel nozzle assembly. Flame holding can result in significant damage to the fuel nozzle assembly, as increased temperatures within the fuel nozzle exceed the design temperatures of the nozzle materials. Additionally, prolonged flame holding may cause the nozzle material to melt away. This can lead to serious damage to the turbine blades, as melted portions of the fuel nozzle assembly flow through a combustor and into the turbine section of a gas turbine.
In order to prevent such damage, various devices have been proposed to detect flashback and flame holding in a fuel nozzle assembly. For example, some detection devices use thermocouples to detect temperature changes. However, thermocouples only provide flashback and flame holding detection at single points within a fuel nozzle assembly. Accordingly, it is quite complex and costly to place thermocouples in every location within a fuel nozzle assembly where flame holding may occur. Other devices are known that utilize an electric field to detect flames within the fuel nozzle assembly. However, this requires electrical wiring running to each nozzle in order to achieve nozzle-level detection. Moreover, it has been found that there are both cost and reliability issues associated with the use of electric fields to detect flames.
Accordingly, there is a need for a system and method for detecting and controlling flashback and flame holding within a combustor that is reliable, relatively simple and effective without being cost-prohibitive.